Enhancing Paper’s Mechanical Strength and Antibacterial Properties through a Biopolymer-Based Coating

This study presents a sustainable approach to improving the antibacterial and mechanical properties of paper through the application of a biopolymer-based coating derived from citric acid-modified soybean flour (SBFC) and carboxymethyl cellulose (CMC). The SBFC was synthesized via thermal cross-linking between citric acid and defatted soybean flour under optimized conditions (140 °C for 2 h, with Na₂HPO₄ as a catalyst), facilitating ester and amide bond formation that was confirmed via FTIR spectroscopy. Aqueous solutions of SBFC were subsequently blended with CMC at varying ratios, and a 50:50 formulation was optimized for the best performance of dispersion stability and coating performance. The final SBFC/CMC coating exhibited a 61.8% improvement in tensile strength compared to uncoated paper. Antibacterial activity was evaluated using Escherichia coli ATCC 25922 as a model Gram-negative bacterium, with bacterial reduction confirmed through CFU-based plate counts. The coating achieved a log₁₀ reduction of 1.2–1.7 relative to that of the controls, corresponding to an 84–89% reduction in viable bacterial colonies. While testing was limited to a single bacterial strain, the results demonstrate promising antibacterial performance. This work highlights the potential of citric-acid-modified biopolymer coatings as multifunctional additives for improving the performance and hygiene properties of paper substrates, particularly in packaging applications.